1. Field of the Invention
The present invention relates to a laminated filter adopted to devices using a radio frequency (RF), such as communication systems and/or broadcasting systems, and more particularly to a laminated filter capable of forming cross capacitive coupling as feeding lines of input lead and/or output lead are coupled to a resonator pattern, enhancing an attenuation characteristic of stop band, being minimized if feeding lines of the input lead and output lead are aligned in different layers, and easily adjusting a position of attenuation pole based on adjustment of feeding line length.
2. Description of the Related Art
Generally, a band pass filter passing signals with a specific frequency band includes a plurality of LC resonators. For example, a prior art laminated filter is depicted in FIG. 1.
FIG. 1 is an exploded perspective view of a prior art laminated filter.
As shown in FIG. 1, the prior art laminated filter is formed to include dielectric cover sheets 11A and 11B laminated atop one another. Dielectric grounding sheets 12A and 12B are stacked on the inner sides of the dielectric cover sheets 11A and 11B, respectively, and form grounding electrodes G1 and G2 thereon, respectively. Three dielectric sheets 13, 14 and 15 are placed between the dielectric grounding sheets 12A and 12B. The dielectric sheet 13 forms input and output feeding lines 13a and 13b to be connected to external input and output electrodes, respectively, placed at both sides thereof. Capacitor patterns 13c and 13d, respectively connected to the input and output feeding lines 13a and 13b, are formed on the dielectric sheet 13.
Also, the dielectric sheet 14 forms first and second parallel resonator patterns Q1 and Q2 thereon, whose lengths are smaller than λ/4 with respect to a center frequency, respectively. The first and second parallel resonator patterns Q1 and Q2 are arranged so as to face the capacitor patterns 13c and 13d, respectively. The first and second resonator patterns Q1 and Q2 are coupled in parallel to one another in a mutual electro-magnetic coupling fashion.
The dielectric sheet 15 forms a coupling capacitor pattern 15a thereon, in which the coupling capacitor pattern 15a forms additional electric coupling between the first and second resonator patterns Q1 and Q2. The coupling capacitor pattern 15a can adjust the amount of mutual coupling of two-pole filter, form an attenuation pole at a stop band, and control the amount of mutual coupling such that the position of the attenuation pole of the stop band can be adjusted.
FIG. 2 is an equivalent circuit diagram of the prior art laminated filter of FIG. 1.
As shown in FIG. 2, IN and OUT correspond to an input lead and an output lead, respectively. C1 and L1 form an LC equivalent circuit of a first resonator pattern Q1. C2 and L2 form an LC equivalent circuit of the second resonator pattern Q2. C3 and L3 form an LC equivalent circuit by the mutual electromagnetic coupling between the first and second resonator patterns Q1 and Q2. Also, C4 is the capacitance between the first resonator pattern Q1 and the capacitor pattern 13c, C6 is the capacitance between the second resonator pattern Q2 and the capacitor pattern 13d, and C6 is the capacitance between both of the first and second resonator patterns Q1 and Q2 and the coupling capacitor pattern 15a. FL1 and FL2 correspond to the input and the output feeding lines 13a and 13b, respectively.
FIG. 3 is graphs illustrating attenuation characteristic of the prior art laminated filter of FIG. 1, in which the graphs are of insertion loss S21 and reflection loss S11 with respect to a center frequency fo of approximately 2.45 GHz. Also, an attenuation pole P1 is shown at 6.8 GHz.
However, in the prior art laminated filter, the coupling of the capacitor patterns 13c and 13d only with a corresponding resonator pattern of the first and second resonator patterns is advantageous in that it restricts enhancement in attenuation characteristics. Also, since the input and output feeding lines 13a and 13b are formed on the same ceramic sheet, the prior art laminated filter has a drawback in that adjustment of the area and length of the capacitor is restricted.
FIG. 4 is an exploded perspective view of another prior art laminated filter.
In the laminated LC filter of FIG. 4, input and output capacitor patterns 72 and 73 are formed on a ceramic sheet 52. Here, the ceramic sheet 52 also forms a coupling capacitor pattern 62 thereon. The input capacitor pattern 72 is arranged so as to face inductor patterns 54a and 54b and is formed by a capacitive coupling in the LC resonator pattern Q1. One end of the input capacitor pattern 72 is connected to an input electrode exposed at the left side of the ceramic sheet 52. The output capacitor pattern 73 is arranged so as to face the inductor patterns 55a and 55b and is formed by capacitive coupling at the LC resonator pattern Q2. The one end of the output capacitor pattern 73 is connected to the output electrode exposed at the right side of the ceramic sheet 52.
The coupling capacitor pattern 62 and the input and output capacitor patterns 72 and 73 are arranged between the inductor patterns 54a, 55a, 54b and 55b formed on the ceramic sheet 52.
As such, since the coupling capacitor pattern 62 and the input and output capacitor patterns 72 and 73 do not block the magnetic field H caused by the inductors L1 and L2, the magnetic field H is uniformly generated such that relatively large inductance can be acquired. Here, reference numerals 60a and 60b denote a shield pattern, reference numerals 58a, 59a, 58b, and 59b denote a capacitor pattern, and reference numerals 56a, 57a, 56b and 57b denote wide width parts connected to the inductor patterns 54a, 55a, 54b and 55b. 
Such a prior art filter has been disclosed in U.S. Pat. No. 6,437,665 B1.
However, similar to the laminated filter shown in FIG. 1, since the prior art filter disclosed in U.S. Pat. No. 6,437,655 B1 is constructed such that the input and output capacitor patterns form capacitive coupling as the input and output capacitor patterns are coupled to corresponding inductor patterns, respectively, it restricts enhancement in attenuation characteristics.
Also, since the input and output capacitor patterns are formed on the same ceramic sheet, the prior art filter has a drawback in that adjustment of the area and the length of the capacitor is restricted.